The present invention relates generally to the visual display devices, and more particularly, to a composite integrated circuit light-emitting display array using LED (light-emitting diode) devices in planar array in a transparent, monolithic crystal wherein the light is generated at the back of the crystal, but is transmitted through the crystal and is observed from the front surface of the crystal.
Heretofore, semiconductor light-emitting devices (whether of discrete, matrix, or array types) have been utilized in a fashion in which light was generated at or close to a "front" surface which is viewed by the observer. It has recently been proposed to construct LED devices from a transparent crystal in which light is generated at the "back" surface of the crystal but is transmitted through it and viewed from the front of the crystal. It is herein proposed to utilize this concept in a composite semiconductor array which includes an integrated circuit. The array disclosed is of an X-Y addressable configuration, i.e., a solid state planar array.
In the past, a variety of devices for non-permanent visual presentation of information has been available. Perhaps one of the most widely used and accepted visual display devices is the cathode ray tube. While the cathode ray tube has served admirably in many diverse applications, it does suffer many disadvantages which limits its future use in many applications. These include high-voltage requirements (usually of the order of 15,000 volts or more) and X-ray and radio frequency emissions which are attendant to such high voltages requiring shielding in order to reduce radio-frequency interference. In addition, cathode ray tubes are expensive owing to their complexity of manufacture and their relatively short lifetime requiring periodic replacement. The low light output of cathode tube displays requires care to prevent glare from the ambient surroundings from preventing the readability of the display. Even under ideal conditions, cathode ray tube displays being analog in nature have poor resolution with accuracies achievable only to about 2%. Furthermore, the large size and weight and the non-ruggedness of the cathode ray displays prevent their use in many environments where space is at a premium and rugged, dependable construction is a prerequisite.
Another type of display is the so-called plasma discharge (or simply "gas discharge") display, in which gas between planar electrodes in a gas-filled envelope is excited by voltages across the electrodes provides emission of light. Such devices require relatively high energizing voltages, are somewhat bulky, and have limited lifetime. Because of their higher energizing voltages, they are not directly compatible with semiconductor circuitry of the type employed in integrated circuits.
These and other types of displays are described, and their relative merits noted, in the article entitled "Circuit/System Building Blocks" by Lapidus, G., in IEEE Spectrum, Vol. II, No. 1, January, 1974, p. 54.
Recently, semiconductor light-emitting devices have been developed. The development of these devices has given rise to the forecast of a solid state planar display that will overcome all of the above limitations of cathode ray tube displays and, perhaps, have many other added benefits. However, while these light-emitting devices lend themselves to X-Y scanning, a simple X-Y matrix of solid-state light-emitting devices has many drawbacks. One major disadvantage to X-Y scanning of a large array which may have of the order of 500,000 light-emitting devices, is that high-peak currents are required for very short period of time. For example, assume that a planar display has 500,000 light-emitting devices in a 7 .times. 7 inches format with 100 light-emitting devices to the inch. A 10 mA DC current is required for each light-emitting device to provide an average display brightness of 1,000 lumens per square foot. For a frame rate of 50Hz, the clock scan-rate must be greater than, or equal to, 25 MHZ. Under these conditions, it can be calculated that the peak current in any light-emitting device in the array will be 5000 A. Even with two orders of magnitude reduction in DC current in the light-emitting devices, the peak current would be approximately 50 A. This is intolerable and presents an almost insolvable problem when one considers the potential radio-frequency interference in addition to the small conductor cross-sectional areas associated with an array of the above dimensions.